include ELIXIR logo in the footer

[bpiereck]

I dont know ho to find or edit the footer file, but i think we should add there the elixir logos since I have added the new logo to reinforce our visual identity

[GeertvanGeest]

Gave it a try, but unless we extend the theme and start writing html ourselves, it will be difficult to include a large logo in the footer: https://squidfunk.github.io/mkdocs-material/setup/setting-up-the-footer/